1. Field of the Invention
The present invention relates to a camera, and more particularly, to a camera in which a photograph is taken after drive control of a photographic lens is performed in accordance with results of distance measurement, and to a camera that regulates movement of the photographic lens along the optic axis.
2. Description of Related Art
With a camera in which a photograph is taken after drive control of the photographic lens is performed, a system has been proposed that detects when the lens barrel is stopped by an external force while the lens barrel is being driven. The stoppage of movement is detected and the supply of electric current to the motor is cut, thereby preventing damage to the drive circuit and to the lens barrel drive mechanism.
With conventional cameras there exists the problem that when an abnormal condition is detected, the lens barrel is stopped and remains stopped. Even after the external or similar force that caused the abnormal condition is removed, it is impossible to continue using the camera until the photographer performs some kind of restoration action.
With conventional types of auto-focus cameras, the current to the lens drive is terminated and the drive sequence is stopped when the lens does not attain a set position within a prescribed time interval. With conventional cameras in which the lens mirror tube, that is, the lens barrel, protrudes from the camera cover and performs focussing and zooming, the lens barrel driving action completely stops if the photographer carelessly touches the lens mirror tube, causing it not to attain a set position within a prescribed time. In this state, photography is not possible, and, in addition, the camera cannot be restored without a complicated operation. Also, if electric current continues to flow and the driving action is not stopped, abnormally high temperatures may be generated in the drive motor, damaging it.
Japanese Patent Publication No. 61-55099 discloses a single focus camera that includes a shutter action control that exposes the proper amount of light to the film, a distance measurement device that measures the distance from the camera to the object being photographed, and a lens barrel drive control that uses a signal from the distance measurement device and drives the lens to the proper position. After the camera measures the distance, if the photographer's hand or another object accidentally touches the lens barrel and prevents it from moving to the target position, the lens barrel drive control detects that the driving action cannot be completed within a prescribed time interval. The lens barrel drive control then terminates the current to the drive motor and stops driving the lens barrel. Even if the lens barrel is not driven, however, the shutter action control still is operable. Thus, a photograph can be taken even if it is out of focus.
On conventional cameras equipped with a zoom lens, to vary the magnification the photographer operates a driving switch on the exterior of the camera body. The lens barrel then is thrust out or retracted by electromotive force. Such cameras equipped with a variable magnification optical system differ from single focus cameras, in that no countermeasures are performed in the event the lens barrel is unable to move to its target position.
In such cameras, therefore, problems can arise if the lens barrel is accidentally touched by the photographer or has an external force applied to it from some other source. When photography is performed through a closed window, for example, the lens barrel may hit the window when the photographer varies the magnification. If the driving switch is left on, the drive motor may overheat or the shutter may not switch off, and photography will not be able to proceed.